Gas generators, i.e., devices for producing gas, have become more common place in the field of pyrotechnics over the last 20 years, mainly due to the increased use of automotive air bags. Typically, an automotive air bag gas generator, which is referred to as an inflator, contains a gas generant, i.e., a pyrotechnic material that generates a gas during combustion. The gas generant in the inflator in a vehicle air bag passive restraint system is typically a pyrotechnic material comprising a fuel and an oxidizer or, in the case of mono-propellants, such as nitrocellulose, a fuel having an integral oxidizer. The gas generant must provide the gas required to deploy and fill the air bag in a matter of milliseconds when an actuation signal is received by the system, and the air bag inflator must perform properly during an accident at any point in the useful life of the vehicle. The fact that an inflator may be required to rapidly fill an air bag after 10 or more years of storage places a number of constraints on inflator design, which are dictated by the required performance of the restraint system, i.e., the time required for the full deployment of the air bag, reliability (including environmental exposure and storage life), the safety and health of vehicle occupants, air bag volume, and the interface between the restraint system and the vehicle. The inflator specification that results from these constraints defines the form, fit, and function criteria for the inflator.
An example of a filterless inflator is provided in parent U.S. Pat. No. 5,551,725, which is incorporated herein by reference. The inflator described in the above identified application comprises a contained volume, a source of gas for producing an inflation gas, an initiating system for initiating the conversion of the source of gas to the inflation gas, and an exhaust orifice that provides an exhaust path and controls the flow of the inflation gas. The source of gas is typically a mixture of a fuel and oxidizer that is stable, and will not ignite until the initiating system ignites the mixture to produce the inflation gas.
A typical inflator functions by converting an electrical or mechanical initiating signal into the generation of a precisely controlled quantity of gas at precisely controlled rates. Generally, this is accomplished by an inflator pyrotechnic train, which comprises an `initiation` device called an initiator, an enhancer charge, and a main gas generant charge, all of which are contained in the body of the inflator. In response to the initiating signal, the initiator ignites and produces a hot gas, particulates, and/or flame. The flame output of the initiator is typically small, and often requires enhancement to ignite the main gas generant charge. The initiator flame ignites the enhancer charge, which is a hot burning propellant, and augments the initiator output sufficiently to ignite the main gas generant charge. Once ignited, the gas generant burns to produce the hot gas required at a rate sufficient to fill the air bag module in the required time.
The restraint system performance is dictated, in part, by the need to fill and deploy the air bag in a matter of milliseconds. Under representative conditions, only about 60 milliseconds elapse between the primary impact of a vehicle in an accident and the secondary impact of the driver or passenger (herein after "an occupant") with a portion of the vehicle interior. Therefore, a very rapid generation or release of gas is required to fill the bag, and prevent the secondary impact. The amount and rate of gas generation or release is determined by the volume of the air bag required for the vehicle and the time between primary and secondary impacts.
In addition, to meet environmental and occupant safety and health requirements, the inflation gas produced by the inflator should be non-toxic and non-noxious when the inflator is functioned in an air bag module in a typical vehicle. The gas generated or released must also have a temperature that is sufficiently low to avoid burning the occupant and the air bag, and it must be chemically inert, so that the mechanical strength and integrity of the bag are not degraded by the gas.
The stability and reliability of an inflator gas generant over the life of the vehicle are extremely important. The gas generant must be stable over a wide range of temperature and humidity conditions, and should be resistant to shock, so that the propellant pellets, grains, granules, etc. maintain mechanical strength and integrity during the life of the vehicle.
Vehicle manufacturers have developed a number of quantitative tests to determine whether an air bag restraint system will operate reliably when needed during any part of a vehicle's useful life. Although these tests and the performance requirements that an inflator should meet in these tests vary somewhat from manufacturer to manufacturer, the design criteria of all the vehicle manufacturers are essentially the same.
In a typical prior art passive restraint system the inflation gas is nitrogen, which is produced by the decomposition reaction of a gas generant containing a metal azide, typically sodium azide (NaN.sub.3). The metal azide is the fuel and the principal gas generating compound in the gas generant used in the inflator. A typical metal azide gas generant is disclosed in U.S. Pat. No. Re. 32,584.
The gas produced in sodium azide based inflators is relatively pure nitrogen. Because there is no carbon in the fuel, oxides of nitrogen, NO.sub.x, can be controlled easily by running the propellant under slightly fuel rich conditions. In contrast, the combustion of gas generants containing carbon, nitrogen, and oxygen, when formulated to be fuel rich, results in the production of carbon monoxide (CO), a toxic gas. If excess oxygen is present in such a composition to assure the complete oxidation of CO to carbon dioxide, the excess oxygen will react with nitrogen at the propellant combustion temperature to form oxides of nitrogen, which can also be toxic. Therefore, the mixture of oxidizer and fuel must approach a stoichiometric balance in gas generants of this type to avoid the production of toxic gases.
Inflator designs based on sodium azide have been shown to meet the requirements of vehicle manufacturers, and are used today in most passive restraint systems. However, there are disadvantages to this technology, including the production of large quantities of hot, solid particulates during combustion, such as sodium oxide, a highly caustic material, which results in added complexity and cost in the inflator design. The relatively high toxicity of the raw sodium azide (oral rat LD.sub.50 of about 45 mg/kg), which must be handled during the inflator manufacturing process, can also create a disposal problem at the end of the useful life of the vehicle.
Because typical gas generants used in inflators produce solid particulates, filters must be incorporated into the inflator to separate the hot particulates from the gas prior to exhausting the gas from the inflator into the air bag. Filters are required in virtually all driver and passenger side air bag inflators that incorporate purely pyrotechnic gas generants, including sodium azide based air bag inflators because of the significant amounts of solids produced during the decomposition of the oxidizer and the combustion of the fuel. The solids produced during the combustion of the gas generant are separated from the gas stream to prevent exposure of vehicle occupants to excessive or toxic levels of airborne particulates during and after air bag deployment. The need for filters, as well as the toxicity of the sodium azide, adds to the cost of producing a typical prior art inflator.
As a result of the problems associated with sodium azide based gas generants, there is movement away from sodium azide based technology to "non-azide" based technology, which uses gas generating compositions, i.e., gas generants, that are typically simple organic fuels, such as hydrocarbons, carbohydrates, or derivatives thereof used in concert with one of the more classic pyrotechnic oxidizer, such as potassium perchlorate, potassium nitrate, or strontium nitrate. These compositions have been used as gas generants in purely pyrotechnic inflators and as gas generants and heaters in hybrid inflators, which incorporate both a pyrotechnic element and stored pressurized gas. The main problem with compositions using these oxidizers is still the copious amount of solids produced by these oxidizers upon combustion. When used with a fuel which does not produce solids, the prior art non-azide gas generants are an improvement over sodium azide based generants, but still require extensive filtration prior to the gas exiting the inflator because of the particulates produced by the oxidizers. This results in an inflator that is larger and more expensive than would otherwise be necessary.
Attempts have been made to use ammonium nitrate with a phase stabilizer as an oxidizer, but, generally, these compositions do not hold up to the extensive thermal cycling that can occur in automotive applications. Ammonium perchlorate mixed with an alkali metal nitrate or carbonate in essentially equimolar amounts has also been used, where the alkali metal salt is added to neutralize the hydrogen chloride, HCl, produced by the combustion of the ammonium perchlorate. The resulting combination produces only 50 to 60 percent of the solids produced by the more traditional oxidizers. When ammonium perchlorate/alkali metal salt based compositions are used with a low solids fuel that requires very little oxygen to burn stoichiometrically, the result is a low solids producing gas generant that requires substantially less or even no filter when used in an automotive inflator. For example, U.S. Pat. No. 5,780,768 discloses a mixture of guanidine nitrate, ammonium perchlorate, and sodium nitrate that produces only about 12.5 percent solids upon combustion. This mixture has been successfully used in a driver side inflator without a filter, and is a dramatic improvement over more conventional technology. However, this composition still requires relatively high pressures to combust. A further reduction of solids produced from the unit, as well as a reduction in the combustion temperature, is also desirable.
"Hybrid" inflators that use stored pressurized gas for part of the inflator gas supply are another means used to control solid particulate production, since smaller amounts of solid particulate producing gas generant can be used to obtain the same inflator gas output. In addition, the stored pressurized gas, which is typically an inert gas mixed with oxygen to supplement combustion and decrease the level of toxics, cools the gas that flows from the inflator, and results in a greater degree of condensation and solidification within the inflator. Thus, the amount of particulates introduced into the air bag and the vehicle interior is reduced.
The combination of greater condensation of solids within the inflator and the reduction in the total amount of solids produced eliminates the need for filters in hybrid inflators. However, hybrid inflators are typically larger and heavier, and have decreased reliability resulting from storing a pressurized gas over the lifetime of the vehicle.
U.S. Pat. No. 5,538,567 discloses a gas generating propellant, which produces nitrogen, carbon dioxide, and steam on combustion, consisting essentially of guanidine nitrate, a flow enhancer, such as carbon black, a binder, such as calcium resinate, and an oxidizer selected from the group consisting of potassium perchlorate and ammonium perchlorate. The production of only nitrogen, carbon dioxide, steam, and minor amounts of hydrogen and carbon monoxide is disclosed. However, only a single composition comprising potassium perchlorate is exemplified. There is no example of compositions incorporating ammonium perchlorate, which produces significant quantities of hydrogen chloride (HCl) during combustion.
U.S. Pat. No. 5,545,272 discloses a gas generating composition consisting essentially of about 35 to 55 percent by weight nitroguanidine and about 45 to 65 percent by weight phase stabilized ammonium nitrate, and may include a flow enhancer or a molding facilitator. The phase stabilizer is typically a potassium salt. Although ammonium nitrate produces clean non-toxic gases, and is free of solids upon combustion, ammonium nitrate has a crystal transition or phase stability problem, resulting from the four phase transitions ammonium nitrate crystals undergo over the temperature range typically experienced in storage. Each of these transitions results in a change of crystal volume, which may cause a slow breakup of propellant grains during thermal cycling from high to low temperature. However, ammonium nitrate crystals can be "phase stabilized" using additives, such as potassium perchlorate and potassium nitrate. The effectiveness of these additives varies depending upon the particular additive used. However, most of the known additives useful as phase stabilizers produce solids upon combustion, and, thus, increase the production of solids by the propellant.
Therefore, a need exists for pyrotechnic materials that can be used as low solids producing gas generants that minimize or eliminate the need for inflator filters or other means for separating solids from the gases produced. The present invention provides such pyrotechnic materials.